WORKING WITH CONCRETE

One of the most versatile of all building materials, concrete is also one of the easiest to work with. The techniques for laying anything from a garden path to a patio are much the same – once you know the basic rules for mixing up the ingredients, making formwork, laying and levelling.

What to buy for mixing concrete

The quantities given here for sand and aggregate are rounded up to the nearest fraction of a cu metre that can be ordered. The mixes are made up by volume so some sand and aggregate may be left over.

Oncrete is made of cement, aggregate and water. Cement itself is quite a complex chemical formed by burning chalk, limestone and clay at high temperatures and then grinding the resulting clinker to a fine powder. Added to the water it becomes an adhesive and coats and binds the aggregate (clean, washed particles of sand, crushed stone or gravel — never brick — for the clays can react against the cement).

The strength or hardness of any concrete simply depends on the proportions of these ingredients. Only a small part of the water you add is used up in the chemical reaction — the rest evaporates.

Ordinary portland cement is used for most concreting work. (The name doesn’t refer to the manufacturer or where it is made, it’s simply that when invented in the 1820s it was thought to resemble Portland stone.)

Aggregates are graded according to the size of sieve the particles can pass through — anything from 10-20mm. Coarse aggregate has the largest stones (20mm) while fine aggregate, often described as shingle, can be 10-15mm. Sand, the third part of concrete, is also considered aggregate. (The cement holds the sand together and the combination of sand and cement holds the stones together.) In concreting the sand used is known as ‘sharp’ sand and graded by the sieve method. All-in aggregate is a combination of both sand and stones.

Choosing your mix

Different projects require different mixes of concrete. Three are most commonly used.

Mix A is a gene’ral-purpose mix for surface slabs and bases where you want a minimum thickness of 75mm-100mm (3-4in) of concrete.

Mix B is a stronger mix and is used for light-duty strips and bases up to 75mm (3in) thick – garden paths and the like.

Mix C is a weaker mix useful for garden wall foundations, bedding in slabs and so on, where great strength is not needed.

The amount of water needed depends very much on how wet the sand and stones in the aggregate are. A rough guide is to use about half the amount (by volume) of cement. But add it gradually. Too much will ruin the mix and weaken the concrete.

How concrete works

New concrete hardens by chemical action and you can’t stop it once it’s started. The slower the set the better and it is important that after laying, exposed surfaces are covered with wet sacks, sand or polythene (and kept wet) for the first 4-6 days. Con-crete also gives off heat as it sets — a useful property in very cold weather, although it would still need covering to protect it from frost. Freshly mixed concrete will begin to set within 1-2 hours — in dry hot weather it will be faster. It takes 3-4 days to become properly hard — you can walk on it at this stage. To reach full strength, however, may take 28 days or more.

How to buy concrete

For small jobs it’s best to buy the cement, sand and aggregate dry-mixed together, in either 10kg, 25kg or 50kg bags. All you have to do is add water.

For larger projects, this can work out to be very expensive. Here it is better to buy the materials separately. The cement is normally sold in 50kg (just under 1 cwt) bags, though smaller (again more costly) quantities – 10kg and 25kg – are available. Both sand and aggregate are sold in 50kg bags, but it is more common to buy them loose by the cubic metre or fraction of a cubic metre. The combined or ‘all-in’ aggregate is available in the same way.

For really large work, however, (patios, long drives and the like) mixing the amount of concrete required by hand is extremely hard work. You could hire a powered mixer, but generally it is more convenient to buy it ready-mixed and have it delivered to the house. Check with the supplier on the minimum amount they are prepared to deliver — for quantities close to that minimum you could find it prohibitively expensive and you should consider sharing a load with a neighbour who is also carrying out building work. With ready-mix remember that you 2 — have to be prepared to lay it fast and if there is no direct access to the site and the concrete can’t be tipped directly into your prepared formwork, you must have plenty of able-bodied help with heavy-duty wheelbarrows (you can hire these) standing by.

Dry-mixes have the amounts that made-up concrete will cover printed on the bag. For mix-at-home quantities, see the ESTIMATOR.

GETTING A LEVEL BASE

This is vital to avoid weak, thin spots in the concrete which will crack. These methods work on a reasonably flat site:.

– drive 300mm (12in) pegs into the ground at 1 metre (3ft) intervals

– align their tops with batten and spirit level to match the final surface level of the concrete

– dig away soil to required depth, taking care not to disturb pegs. Use amount of peg exposed as your depth guide

To level top of pegs over longer distances and round corners:.

– tie a length of transparent hose between pegs

– fill the hose with water and drive in pegs so their tops match the water levels either end

On long paths and drives:

Use sighting rods made from sawn softwood – each is the same height (about 1.2 metres/4ft) and has a tee piece exactly at right angles across the top – you’ll need three rods, and two helpers. Place rods on pegs and line up tops by adjusting pegs in ground.

FOUNDATIONS FOR CONCRETE.

– as a general rule lay rammed hardcore to the same depth as the final concrete

– on soft sub-soil excavate to twice the depth, filling soft pockets carefully with extra hardcore

How to store materials

Under normal conditions cement will start to harden after about 30 days simply because it’ll be absorbing moisture from the air. However, older cement that’s still powdery inside can still be used where great strength or a high quality finish is essential – but mix in a higher proportion of cement than usual, ie 1:1:2.

Cement should always be stored under cover and raised well off the ground — on a platform of wood, for example. Stack the bags closely, keep them clear of other materials and cover them to help keep the moisture out. If a bag has been part used, the remainder can be stored for a while inside a well-sealed plastic bag.

Loose sand and aggregate should be pijed on a flat, dry and hard area and covered with heavy-duty plastic sheeting. It’s most important to avoid the aggregate being contaminated by soil or other foreign materials. Any organic matter would de-compose in the concrete leaving ‘voids’ which weaken it.

Site preparation

This is a major stage before you begin to erect any formwork, mix or lay any concrete. For accurate marking out, use pegs and strings to give yourself guide lines to follow. The area should be dug out and made as level as possible in the prepared area.

Is another way of describing land that’s been reclaimed. There’s no knowing what was used as the in-fill, and it should always be assumed that it has minimal load-bearing capacity. Any concreting here will need good reinforcement such as hardcore, well compacted and the same thickness as the concrete you’re laying on top.

Soft pockets

After you’ve prepared a site for laying a path or patio you could find pockets of soft soil which will cause any concrete to sink.

Large areas of soft pockets or made-up ground need something solid as a base — and this is where hardcore (broken concrete), rubble (broken brick) or a very coarse aggregate is essential. Tamp it into the ground until well consolidated — a must for areas such as drives or structural foundations taking a lot of weight.

If necessary small areas can be reinforced with a steel mesh set into the concrete. For most purposes 7mm diameter rods formed in a mesh of 150mm squares is quite adequate, and this is readily available at most builders’ merchants. Rest the rods on small pieces of broken brick before you lay the concrete; make sure that the ends of the rods don’t protrude from the area you’re concreting, and that the mesh is completely covered.

Creating the work area

Using formwork boards to create a kind of box in which to lay your concrete has two big advantages. Firstly, it contains the concrete neatly, and secondly it gives you levels on either side to guide you in levelling the concrete itself. Although this is the most usual method of containing concrete, a brick surround can be used just as well – and this has the added advantage of not having to be pulled up. With bricks, however, it’s more difficult to establish a completely straight and level line to follow.

For formwork use sawn (unplaned) softwood – it’s called carcassing in the trade – for concrete that’s to be placed below ground. It should be as wide as the depth of concrete you intend to lay and 25mm (1 in) thick. Don’t skimp on the thickness for it must be firm and rigid to support the weight of concrete.

Pegs are used to keep the formwork in place. They must be sturdy, not less than 50mm (2 in) square and long enough to go well into the ground. Place pegs every 1 metre (3ft) against the face of the boards.

If building a raised path, formwork will give a finish to the concrete edge so you should use a timber that’s planed. Unplaned timber can be used if the formwork is lined with 6mm Cain) plywood, or if you intend finishing off the edges with more concrete after the form-work has been removed.

If you want to curve a corner in the formwork, use hardboard cut into strips as wide as the concrete is deep. This will need to be supported with pegs at more frequent intervals than softwood boards.

If you have difficulty driving the pegs into the ground (which may happen if you’ve put down hardcore) use lengths of angle iron instead. Alternatively drive the pegs in further away from the formwork and put timber blocks between the peg and formboard.

Expansion joints

Any large area of concrete needs expansion’ or ‘movement’ joints or it is liable to crack up. A one piece slab shouldn’t be more than 4 metres (13ft) in any direction without a joint being included. The simplest method to use is to incorporate a length of flexible plastic movement joint which can be bought at builders’ merchants. Or you can set in a piece of soft board the same depth as the concrete and about 12mm (£in) thick which can be removed and the space filled with bituminous mastic – this doesn’t give an altogether attractive finish however. If new concrete is to join an existing slab, use a piece of thick bituminous felt for the joint.

Drainage slopes

With a wide expanse you should have a gentle slope (1 in 60 is the general rule) so that rainwater can drain away. This is achieved by setting the forms on one side slightly deeper into the ground. To check that the slope is the same all along the formwork, set a small piece of wood (about 12mm/}in for a 1 m/3ft wide path) thick on the lower side and use your spirit level to check across to the other side.

To keep the formwork on each side of a path rigid, place a length of softwood across the width at the peg points, but not so that it will make an impression on the concrete. This can be used as a guide for levelling as well.

Whether you are building a concrete path, a base for a shed or garage, a hardstanding for a car or even a large patio, the principle of formwork is the same – only the number of boxes or bays you divide the area into varies. With each stage of the job you should mix only enough concrete to fill one bay or box at a time.

As the concrete starts to dry (after 2 hours) cover the surface with plastic sheeting or damp sacking to protect it.

Building a Concrete Patio

Tools and materials

Formwork boards – 100mm x 25mm (4in x 1 in) bought in 3 metre lengths Concrete – the area of patio (excluding path) was worked out as though it was a circle. The radius (half the diameter) was 1.25 metres so the area was just under 5 sq metres -(3.142 xR2). The volume of concrete needed was calculated by multiplying the area by the depth. This was 100mm (4ins) so the volume was 5×0.1 =0.5cu metres.

Tamping beam- madeof 150mm x 50mm timber (6in x 2in) timber. Panel saw Club hammer Shovel Spirit level Strings and pegs Claw hammer and nails

A small concrete patio demonstrates not only how easy it can be to work with concrete, but also the versatility of formwork that contains it.

Formwork for paths and patios is really no more than a box. First it gives you a clean edge to your shape, second it gives you sides that make tamping down and levelling easy. Dealing with a long path may mean you have to subdivide the box into smaller bays if you only have limited time and need to spread the work over several days or weekends. Each bay can be finished off with an expansion joint, and the next one started whenever it’s convenient.

The small patio illustrated here was laid in one go. Because of its width (just over 2.5 metres/8ft at its widest) it could have been divided into two bays, which would have made tamping slightly easier and manageable by one person. Instead, it was treated as a single box’ and so a much wider tamping board was used. This meant that two people had to do the tamping, one at each end of the board.

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